Bone anchoring apparatus and method

ABSTRACT

A bone anchoring assembly and method for attaching a body part to a patient&#39;s bone which includes a bone anchor that is implantable in bone having an opening leading to a cavity formed therein. A coupling member with at least one portion made of a shape memory material having a transformation temperature range. The coupling member is movable through the opening and in and out of the cavity at temperatures below the transformation temperature range. With it in the cavity and at temperatures above the transformation temperature range, the coupling member is locked within the bone anchor. The coupling member is attachable to the body part.

FIELD OF THE INVENTION

The present invention is related to a bone anchoring system, and moreparticularly to an apparatus and method for anchoring a body part tobone.

BACKGROUND OF THE INVENTION

In a number of medical procedures, one part of a patient's body isanchored to a desired location on one of the patient's bones. The bodypart being anchored may be living tissue, such as a ligament or bone, ora prosthetic replacement of an original body part. In one suchprocedure, a dental prosthesis, such as a crown or bridge, is used toreplace one or more of a patient's teeth. Such dental prostheses havebeen anchored to bone using an anchoring assembly which included a boneanchor, such as a female bone screw or other endosseous implant, forbeing embedded in the patient's jaw bone. One such bone anchor has anopening leading to a threaded bore and a coupling member adapted at oneend for being threaded into the bone screw bore. In another anchoringassembly, the coupling member is cemented into the bore formed in thebone anchor. The other end of the coupling member is fixed to the dentalprosthesis. Sometimes the crown fractures off of the tooth leaving itsroot intact and embedded in the patient's jaw bone. In such situations,the tooth root may be used as the bone anchor, with the coupling membercemented or otherwise fixed within the tooth's root canal. One problemwith such anchoring assemblies is that if the coupling member has to beremoved, such as to replace the dental prosthesis, a force must beapplied to the coupling member which is transmitted to the bone anchor.For example, if the coupling member is threaded into the bone anchor, asufficiently high torquing force must be applied in order to unscrew andremove the coupling member. If the coupling member is cemented to thebone anchor, a sufficiently high force must be applied to break thecement bond between the coupling member and bone anchor.

Such female bone screws, or other bone anchors used with a couplingmember, are preferably first implanted and left in the patient's bonefor a period of time before the coupling member is secured to the boneanchor and an external force is applied to the bone anchor. This is doneto permit the bone time to grow and bond with the bone anchor, therebyincreasing the strength of the anchorage to the bone. The bone is lesslikely to bond to the bone anchor if an external force is applied soonafter the bone anchor is implanted. Even if this bond is allowed tostrengthen, the force required to remove the coupling member may be sohigh as to cause any bond between the bone anchor and the bone to weakenor even fail. In addition, screwing the threaded coupling member intothe bone anchor may also apply a force of sufficient magnitude to weakenor even break the bond between the bone anchor and the bone. If thisbond weakens or fails, the coupling member will likely need to beremoved from the bone anchor in order to limit any external forceapplied to the bone anchor. Limiting such external forces is necessaryin order to increase the likelihood that the bone-to-anchor bond willincrease in strength. This additional delay in anchoring the body partto bone may inconvenience and cause additional discomfort to thepatient.

Other problems associated with such anchoring assemblies may include therisk of the bone fracturing during removal of the coupling member fromthe bone anchor. This may also occur during insertion, particularly whena threaded coupling member is used. When the bone anchor is implanted, acomparable volume of bone is removed essentially leaving a cavity whichmay act as a stress concentrator. For smaller bones, such as some jawbones, the amount of bone removed may significantly weaken the remainingbone at the anchoring site, making the bone more susceptible tofracturing when insertion or extraction forces are applied to thecoupling member. In addition, when threaded coupling members are used,there is the risk that the threads will bind up. If this occurs, greaterforces may need to be applied in order to insert and extract thecoupling member. These greater forces may in turn cause the couplingmember to fracture, typically at the threads. In order to subject thepatient to as little discomfort as possible, it is typically desirableto complete the replacement procedure as quickly as possible. However,if there are spatial constraints, such as those associated with thesmall confines of a patient's mouth, attaching and removing the couplingmember to and from the bone anchor may take more time than is desirable.

Other bone anchoring systems have been used to attach soft body tissueto a patient's bone. For example, certain orthopaedic surgicalprocedures require that a ligament be reattached to bone. Such ligamentreattachment may occur, for example, in the knee or elbow where spatialconstraints are typical. Such soft tissue attachment has typically beenaccomplished in the past by directly securing the soft tissue to thebone with a suture. Various types of bone anchors, such as bone screws,have been used in the past to anchor the suture to bone. These anchorsare generally made of bio-compatible stainless steel or similarnon-corrosive metal such as titanium or chromium-cobalt alloys. Suturethread is attached to the anchor and the anchor is implanted directlyinto the bone.

When a patient's soft tissue, such as a torn ligament, must bereattached to bone, the torn end must be anchored to the bone and placedin close proximity to allow the ligament to grow into the bone mass andreattach itself. The closer the suture is anchored to the original pointof attachment of the ligament, the better for body mechanics. Designlimitations of prior art devices have been known to limit the optimalplacement of the suture anchor and have lead to less efficientattachment of ligaments. Manipulation required to set the anchor intobone may impact upon the ability of the anchor to be used in locationshaving spatial constraints. The degree of manipulation required mayprevent the use of less invasive surgical techniques, like arthroscopicsurgery, and require the use of a more invasive technique to ensure thatthe anchor can be implanted at the optimal location. Some prior artanchors may just be unsuited for use in areas having spatialconstraints. If a bone anchor is unsuitable for placement in theoriginal location of ligament attachment, a nearby location havingsuitable space must be utilized. Another problem with such an anchoringdevice is that if the soft tissue ever detaches from the bone anchor orif the suture breaks, the screw may have to be removed and implantedagain. Such reimplantation causes additional trauma to the bone whichmay ultimately result in the original bone anchoring site beingunsuitable for subsequent anchoring.

Various anchoring systems have also been used to attach bone-to-bone,such as through the use of a bone plate, like that used in distractionosteogenesis devices or bone fixation plates. Typically, such devicesare anchored to bone using screws or some other anchor implanted in thebone. These prior bone anchors typically must be removed from the boneif the device they are anchoring is to be removed or replaced. Again,the trauma to the bone caused by such reimplantation may result in theoriginal bone anchoring sites becoming unsuitable for subsequentanchoring.

Therefore, there is a need for an anchoring system which enables a bodypart to be fixed to and detached from a patient's bone whileeliminating, or at least substantially limiting, any additional traumato the bone. More particularly, there is also a need for an anchoringsystem which if the body part is detached and reattached to the bone,the bond between the implanted anchor and the bone will not besubstantially affected. There is also a need for such a bone anchoringsystem which may be anchored at locations on the bone that have spatialconstraints using surgical procedures that are less intrusive andtraumatic to the patient's body.

SUMMARY OF THE INVENTION

The present invention is directed to a bone anchoring assembly andmethod for attaching a body part to a patient's bone that enables thebody part, whether prosthetic or the patient's, to be quickly attachedto and removed from a particular location on the bone, even locationswith spatial constraints. The present bone anchoring assembly and methodalso make it easier for the body part to be attached to and removed fromspatially constrained locations on the bone with less intrusive surgicalprocedures and thus less trauma to the patient. Furthermore, the presentbone anchoring assembly and method enable the body part to be attachedto and removed from the same location on the bone with no substantialtrauma to the bone in that location.

The anchoring assembly of the present invention includes a bone anchorthat is implantable in bone. The bone anchor has an opening leading to acavity formed therein. A coupling member, such as a pin, with at leastone portion made of a shape memory material having a transformationtemperature range (TTR) is used to couple the body part to the boneanchor. The one portion of the coupling member is movable through theopening and in and out of the cavity of the bone anchor when it is at atemperature below its TTR. With it in the cavity and at a temperatureabove its TTR, the one portion of the coupling member interlocks withthe bone anchor. Preferably, the coupling member is insertable into andextractable from the cavity of the bone anchor with substantially zeroinsertion and extraction forces, respectively, when the one portion ofthe coupling member is at a temperature below its TTR.

Shape memory materials are generally relatively weak and pliable atleast when the material is at a temperature below its TTR and relativelystrong with superelastic properties at least when the material is at atemperature above its TTR. The properties of a given shape memorymaterial typically vary within its TTR. Generally, the strength andsuperelastic characteristics tend to increase toward the hightemperature end of its TTR and decrease toward the low temperature end.The characteristics of shape memory materials are well documented. Forexample, see the following published works: a book entitled EngineeringAspects of Shape Memory Alloys, 1990, published by Butterworth &Heinemann and edited by T. W. Duerig, K. N. Melton, D. Stockel and C. M.Wayman (ISBM No. 0-750-61009-3), including articles therein entitled "AnIntroduction to Martensite and Shape Memory" by C. M. Wayman and T. W.Duerig, pages 3-20; and "The Mechanical Aspects of ConstrainedRecovery", by J. L. Proft and T. W. Duerig, pages 115-129, each of whichare incorporated by reference herein in their entirety. The uniqueproperties of shape memory materials enable any structure made of such amaterial to have one geometric configuration at a temperature below itsTTR and another geometric configuration at a temperature above its TTR.For purposes of the present invention, the working or ambienttemperature of the anchoring assembly is typically a range oftemperatures, preferably those temperatures normally found in the humanbody. The TTR of any shape memory portion of the present coupling membermay fall below or overlap the working temperature(s) of the anchoringassembly.

Thus, the shape memory portion of the coupling member that interlockswith the bone anchor may be processed or trained, according towell-known techniques, to have a shape which can be easily inserted intoor removed from the cavity of the bone anchor with little, if any,applied force when the shape memory portion is at least at a temperaturebelow its TTR. Likewise, in order to lock the one shape memory portionof the coupling member and the bone anchor together, thereby anchoringthe body part to the bone, the shape memory portion may be processed tohave a shape which would not be removable from the bone anchor cavity atleast at temperatures above its TTR without an excessive amount of forcebeing applied. For example, the anchoring assembly may be designed suchthat the bone anchor cavity confines and prevents the coupling memberfrom changing its shape. This in turn would cause strain to develop inthe shape memory material and thereby produce the forces which must beovercome in order to extract the coupling member from the bone anchorcavity. It is also envisioned that instead of or in combination with thegeneration of such strain forces, the shape memory portion of thecoupling member may be trained to have a shape which mechanicallyinterlocks with a mating portion of the bone anchor cavity. For example,matching threads may be used which only engage when the anchoringassembly is at its working temperature(s). In this way, the amount offorce needed to extract the coupling member out of the cavity of thebone anchor at a given temperature may be controlled by the shapetraining of the shape memory portion.

The amount of force needed to extract the coupling member from the boneanchor cavity may not only be controlled by the shape memorized but alsoby the shape memory material selected and its mechanical properties. Byselecting a shape memory material having a TTR that falls below theworking temperature of the anchoring assembly, the strength andsuperelastic characteristics of the material, and therefore the strengthof the locking force, may be maximized. Lower locking forces may beobtained by choosing a shape memory material having a TTR that overlapsthe working temperature(s) of the anchoring assembly. In someapplications it may be desirable for the body part to detach from thebone by extraction of the coupling member from the bone anchor ratherthan risking damage to the body part, such as with soft tissueattachment. The desired locking force may also be obtained by varyingthe mechanical properties of the shape memory material, such as bymaterial selection or processing. The mechanical properties of manyshape memory materials may be varied to a significant degree,particularly shape memory metal alloys.

The coupling member is connectable or attachable to the body part. Thecoupling member may be attached to the body part directly, such as by asuture threaded through an eyelet formed in the coupling member. It mayalso be attached to the body part indirectly, such as through a femalereceptacle, itself attachable to the body part, having an openingleading to a cavity formed therein. With the latter embodiment, thecoupling member may have another shape memory portion which has beentrained to also be movable in and out of the female receptacle at leastat temperatures below a TTR, and while in the female receptacle, trainedto be locked thereto when at least at temperatures above its TTR. Thepresent coupling member may be made from different shape memorymaterials processed such that it is removable from and locked to thebone anchor and female receptacle at different temperatures. Theportions of the coupling member which mate with the bone anchor and thefemale receptacle may also be made from the same shape memory materialhaving the same TTR. In either case, each portion may be separatelycooled below and warmed above its TTR in order to selectively release orlock the coupling member from or to either the bone anchor or the femalereceptacle. In addition, the coupling member and female receptacle maybe attached to one another by using the superelastic characteristics ofthe shape memory material to snap fit, or otherwise, connect the twotogether. The coupling member and female receptacle may also beconnected together using well known techniques and structure such asmating threads, welding or other bonding techniques.

Because the coupling member may be lockable in and removable from thebone anchor implanted in the bone with little, if any, applied forcesimply by changing the temperature of the applicable shape memoryportion, the embedded bone anchor can remain undisturbed if the bodypart has to be detached from the bone, thus optimizing the strength ofthe anchorage. In addition, because the coupling member may be insertedand extracted from the implanted bone anchor with little if anyinsertion and extraction forces being applied (i.e., simply by pushingit in and pulling it out of the bone anchor cavity), the body part maybe anchored to locations on the bone having spatial constraints whichmight limit the applicability of more complex anchoring systems orotherwise require more invasive surgical procedures and cause additionaldiscomfort or trauma for the patient. Furthermore, because it iscontained in the bone anchor, the shape memory portion of the couplingmember does not directly contact body tissue and is therefore morelikely to be compatible with a greater number of patients. Some patientshave been known to have undesirable reactions to certain shape memorymaterials, notably the nickel in nickel-titanium shape memory alloys.

The above and other aspects of the present invention will becomeapparent upon consideration of the following descriptions taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially sectioned side view of one embodiment of thepresent anchoring assembly for attaching a dental-prosthesis to a jawbone in an unlocked condition;

FIG. 2 is a partially sectioned side view of the anchoring assembly ofFIG. 1 with its coupling pin locked in place;

FIG. 2A is a side view of how the coupling pin of FIG. 2 would appear ifnot physically constrained;

FIG. 2B is a side view of how an alternative embodiment of the couplingpin of FIG. 2 would appear if not constrained;

FIG. 2C is a side view of how another alternative embodiment of thecoupling pin of FIG. 2 would appear if not constrained;

FIG. 2D is a side view of how a fourth embodiment of the coupling pin ofFIG. 2 would appear if not constrained;

FIG. 2E is a side view of a fifth embodiment of the coupling pin of FIG.2;

FIG. 3 is a partially sectioned side view of a finger joint prosthesisincorporating the principles of the present invention;

FIG. 4 is a partially sectioned and schematic side view of a distractionosteogenesis device incorporating the principles of the presentinvention;

FIG. 5 is a partially sectioned side view of a modification of theanchoring assembly embodiment of FIG. 4;

FIG. 5A is a partially sectioned side view of an alternativemodification of the anchoring assembly embodiment of FIG. 4;

FIG. 6 is a partially sectioned side view of an adjustable bone plateincorporating the principles of the present invention;

FIG. 6A is a partially sectioned enlarged view of the circled area 6A ofFIG. 6;

FIG. 7 is a partially sectioned side view of one embodiment of thepresent anchoring assembly for attaching a body part to bone using asuture;

FIG. 8 is a partially sectioned disassembled side view of the sutureanchoring assembly of FIG. 7 and a bone anchor implantation tool;

FIG. 9A is a partially sectioned side view of a modification of thesuture anchoring assembly of FIG. 8;

FIG. 98 is a sectional view of FIG. 9A taken along lines 9B--9B;

FIG. 10A is a perspective view of an alternative coupling pin for thesuture anchoring assembly of FIG. 8 in either a cooled or constrainedcondition;

FIG. 10B is a perspective view of the coupling pin of FIG. 10 in awarmed and unconstrained condition;

FIG. 10C is a perspective view of another alternative coupling pin forthe suture anchoring assembly of FIG. 8;

FIG. 11 is a partially sectioned side view of another embodiment of thepresent anchoring assembly for attaching a prosthesis; and

FIG. 11A is a partially sectioned side view of a modification of theanchoring assembly of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

The principles of the present invention shall herein be described indetail with regard to a variety of embodiments and applications of thepresent bone anchoring assembly.

Referring to FIGS. 1, 2 and 2A, one embodiment of the present anchoringassembly 20 is particularly useful for anchoring a prosthetic devicelike a tooth 21 (shown in phantom), such as in the form of a crown orbridge, to a patient's jaw bone 22. The anchoring assembly 20 includes abone anchor 26, such as a root screw, having an opening 27 formedthrough a top surface 28 and leading to a bore or cavity 29 formedtherein. For a patient with the root of the tooth intact and in place(i.e., only the crown fractured off), it may be possible to use theexisting root as the bone anchor 26, with its root canal functioning atleast as a guide for the cavity 29. The bore 29 has an inside dimensionor diameter D₁. The root screw 26 is adapted for implantation in the jawbone 22 such that the top surface 28 is below the patient's gums 32. Atemperature sensitive coupling pin 34 is used to fix a female receptacleor stump 35 mounting the prosthetic tooth 21 to the anchor screw 26. Atleast one portion, and in this embodiment all, of the coupling pin 34 ismade of a shape memory material having a transformation temperaturerange (TTR). The coupling pin 34 has a length L and an outer dimensionor diameter D₂. The mounting stump 35 has its own opening 37 formedthrough a lower surface 38 and leading to a cylindrical bore or cavity39 having an inside dimension or diameter preferably identical to thediameter D₁ of the bore 29 in anchoring screw 26.

Briefly, the working temperature or temperature range (e.g.,temperatures normally found in the human body) of any shape memoryportion of the present invention may be within or above the TTR of theshape memory material(s) being used. The TTR for many shape memorymaterials, especially shape memory metal alloys, can often be adjustedto compensate for a particular ambient temperature. For someapplications, it may be preferable for each shape memory portion of thepresent invention to have a TTR which falls below the workingtemperature(s). In other applications, it may be preferable for the TTRto overlap the working temperature(s).

While the present invention is not to be limited to any particular shapememory material, shape memory metals, particularly binary alloys oftitanium (Ti) and nickel (Ni) as well as Ni and Ti alloyed with, forexample, niobium (Nb), iron (Fe), cobalt (Co), copper (Cn) or manganese(Mn), are of particular interest. With respect to most shape memorymetals, two basic crystal structures exist, martensite below the TTR andaustenite above the TTR. A combination of both structures, to varyingdegrees, may exist within the TTR. Generally, martensite is relativelyweak and pliable, and austenite is relatively strong with superelasticproperties. If the shape memory metal is in the martensitic stage, itwill need to be heated to transform to austenite. If the shape memorymetal is in the austenitic stage, it will need to be cooled to transformto martensite. When the ambient temperature is within the TTR, theactual structure of the shape memory metal is dependent on itstemperature history. When the structure of the shape memory metal is notall but mostly austenite, it can still exhibit superelastic properties.When the structure of the shape memory metal is all or mostlymartensite, it may be comparatively weaker and more pliable than theaustenite structure, but it can still exhibit some elastic properties(i.e., strained to produce an applied force). Some shape memory metals,commonly known as wide hysteresis alloys can have a wide TTR. With thesematerials, it is possible to use the martensitic structure duringinsertion at ambient temperature, thereby enabling adjustment withoutcooling. Once the adjustment is completed, the memory material istemperature cycled (typically by heating above the TTR) to drop themartensite-to-austenite transformation to a temperature well belowambient temperature. The locking effect would therefore be stable, at awide range of ambient temperatures. This type of shape memory materialis particularly useful when the environment is inside a patient's bodywhere repeated adjustment is often not necessary or desirable butstability is.

For the embodiment of FIGS. 1 and 2, the length L of the coupling pin 34is sufficient to extend into the cylindrical bore 29 and 39 of theanchoring screw 26 and the mounting stump 35, respectively, at the sametime. The shape memory coupling pin 34 is longer and thinner at leastwhen it is at a temperature below the TTR of the shape memory materialand, when it has no spatial constraints (see FIG. 2A), it is shorter andthicker at least at temperatures above the TTR of the shape memorymaterial used. This variation in shapes of pin 34 may be accomplished inthree basic ways. The coupling pin 34 may be processed to exhibit a2-way shaped memory, in which case simply cooling or heating pin 34 willcause it to change shape. Pin 34 may also be processed to exhibit onlyone-way shape memory. This would require the shape memory portion of thecoupling pin 34 to be deformed into the desired geometric configurationat a low temperature and then warmed to convert it back to its originalgeometric configuration (see FIG. 2E and the associated description). Asa third method, a wide hysteresis shape memory material may be usedwhich would remain stable even at ambient temperature and would not lockin place until warmed above the ambient temperature.

By making the outer dimension D₂ of the coupling pin 34 smaller in sizethan the inside diameter D₁ of the boreholes 29 and 39 at temperaturesbelow its TTR, the pin 34 may be inserted into and extracted from eitherborehole 29 and 39 with little if any (i.e., substantially zero)insertion and extraction force, respectively, at such temperatures (seeFIG. 1). The anchoring pin 34 may also be trained so that its outerdimension D₂ is larger than the inside diameter D₁ of the bores 29 and39 at temperatures above the TTR of the shape memory material (see FIG.2A). In this way, with the anchoring pin 34 being at a temperature aboveits TTR and disposed in both bores 29 and 39, the pin 34 may be lockedto both the anchoring screw 26 and the mounting stump 35, and theprosthetic tooth 21 may be anchored to the patient's jaw bone 22. Theforce exerted between the coupling pin 34 and the anchoring screw 26 andmounting stump 35 may be varied by changing the outer dimension D₂ ofthe pin 34 reached at the working temperature range in which theanchoring assembly 20 is used. For example, forces as high as 500newtons have been required to pull a shape memory pin (normally havingan unconstrained diameter of 1.43 mm) out of a bore having a diameter of1.4 mm. With this exemplary system, the shape memory pin was insertedinto the bore a depth of about 2 mm. The shape memory material used forthe pin was a TiNiNb wide hysteresis alloy. At temperatures below theTTR, the pin had an outside diameter of 1.39 mm.

With the three elements 26, 34 and 35 sharing the same centrallongitudinal axis 42, it may be difficult to implant the bone anchor 26such that the prosthetic tooth 21 is properly located relative to thesurrounding teeth (not shown). Therefore, it may be desirable to skewthe central axis 42 of the stump 35 relative to that of the coupling pin34 to facilitate positioning of the prosthetic tooth 21 to a desiredlocation relative to the patient's other teeth (not shown).

It may not only be desirable for the expansion of coupling pin 34 inbores 29 and 39 to prevent separation of the stump 35 from the anchoringscrew 26, it may also be desirable to prevent rotation of the stump 35relative to screw 26, up to an ultimate applied force. To aid thelocking forces exerted by coupling pin 34 in accomplishing this task, amechanical locking system may be employed. For example, the pin 34 andeither or both bores 29 and 39 may be given matching non-circularcross-sections (e.g., square, oval, etc.). Rotation of the stump 35 mayalso be prevented by using multiple pins 34 with a corresponding numberof matching bores 29 and 39. The respective top and bottom surfaces 28and 38 of the anchoring screw 26 and mounting stump 35 may also beroughened, such as with mating grooves and ridges (not shown) in orderto provide an additional impediment to relative rotation between thesetwo elements 26 and 35 of the anchoring assembly 20. This may also bedone with the engaging surfaces between the coupling pin 34 and thebores 29 and 39 to prevent not only such relative rotation but also thestump 35 being pulled away from the anchoring screw 26. For example, seethe mating threads shown in FIG. 5. Coupling pin 34 and stump 35 mayalso be joined by means other than those previously described. Forexample, instead of using the shape memory properties of the shapememory material, its superelastic characteristics may be utilized.Structures similar to that shown in FIGS. 2B-D (see description below)may be used with the superelastic properties to snap fit or otherwiselock the coupling member 34 and female receptacle 35 together. They mayalso be threaded together, welded or otherwise bonded together or be ofsingle piece construction.

The principles of the present invention incorporated in the anchoringassembly 20 may also be applied to other prosthetic devices includingany body joint, such as a hip joint prosthesis. For example, the boneanchor 26 could be a femoral stem and the female receptacle 35 could bea femoral head adapted for mating with an acetabular cup component ofthe hip joint prosthesis. Another application may be the anchoring of aprosthetic ear.

Referring to FIGS. 2B-D, alternatively geometric configurations whichcould be utilized for locking the coupling pin 34 into either or bothcavities 29 and 39 may include an aperture 45 formed through the pin 34which is relatively narrow at temperatures below its TTR, such that theouter dimension D₂ of the pin 34 is smaller than the inside diameter D₁of the corresponding cavity 29 or 39. At temperatures above its TTR, theaperture 45 would then be substantially wide enough to cause the outerdimension D₂ of coupling pin 34 to be larger than the inside diameter D₁(see FIG. 2B). The coupling pin 34 may also utilize a forked portion 47having two or more prongs 48 which open up at temperatures above theirTTR to a dimension D₂ greater than the inside diameter D₁ (see FIG. 2C).Coupling pin 34 may also be trained to have a central longitudinal axis49 that is substantially straight at temperatures below its TTR andsubstantially curved at temperatures above its TTR to lock the pin 34 inthe cavities 29 and 39 (see FIG. 2D). This modified pin 34 may betrained to have two or more bends in its axis 49 along its length L. Ingeneral, training a shape memory element to have a two-way shape memoryis more difficult than training the material to have a one-way memory.However, the anchoring assembly 20 may be modified to include a tubularbiasing sleeve 50 which enables the coupling pin 34 to effectively havea two-way shape memory even if it was trained to only have a one-waymemory (see FIG. 2E). The biasing sleeve 50 applies a compressive forceagainst pin 34. Thus, at temperatures below its TTR, the biasing sleeve50 is intended to reduce the outside diameter of pin 34 enough that itand sleeve 50 may be inserted into the cavity 29 of bone anchor 26.However, when the shape memory material warms to a temperature above itsTTR, pin 34 is able to exert enough force to counter these compressiveforces and expand biasing sleeve 50 outward, thereby locking itself inbone anchor 26. Preferably, a longitudinal split 51 is formed throughthe wall of biasing sleeve 50 in order to facilitate this two-way shapememory effect.

Referring to FIG. 3, instead of prosthetic tooth, a prosthetic bodyjoint, such as a finger joint 53, may be attached between two separatebones 54 and 55 using two anchoring assemblies 58 incorporating theprinciples of the present invention, one for each bone 54 and 55. Eachfinger joint anchoring assembly 58 includes a bone anchor 59 and acoupling pin 60 having a shape memory portion 61 at one end disposedwithin the bone anchor 59 in the same manner as previously described foranchoring assembly 20. The coupling pin 60 also includes a portion 62extending out of bone anchor 59 which is attached to one end of thefinger joint 53 by any acceptable means. For example, portion 62 of eachcoupling pin 60 may have an appropriate shape necessary to adequatelymaintain each portion 62 disposed inside the finger joint 53. Portion 62may also be bonded to joint 53. Portion 62 of each anchoring assembly 58may also be made of a shape memory material, with either or both of itsshape memory and superelastic properties being used to attach it to thefinger joint 53.

Referring to FIG. 4, instead of two bones being attached to one anotherthrough a prosthetic body joint, a distraction osteogenesis device 66may be anchored between two segments 67 and 68 of bone using twoanchoring assemblies 70 incorporating the present invention. Eachanchoring assembly 70, similar to the previous anchoring assemblies 20and 58, includes a bone anchor 73, each embedded in its correspondingbone section 67 and 68, and a coupling pin 75. Each coupling pin 75 hasone portion 76 made of a shape memory material having a TTR disposedwithin the bone anchor 73, similar in manner to that previouslydescribed. Each coupling pin 75 also includes another portion 77extending out of the bone anchor 73 which is adapted for attaching to abone plate component 80 of the distraction osteogenesis device 66 by anyappropriate means. For example, each portion 77 may be in the form of ashank with a head 78. Each coupling pin 75 is disposed through a hole 79formed in its corresponding bone plate 80. Each bone plate 80 ispositioned between its corresponding coupling pin head 78 and boneanchor 73. In this way, each bone plate 80 is secured to itscorresponding bone section 67, 68 when the shape memory portion 76 ofpin 75 is locked within bone anchor 73. Each of the bone plates 80 maybe anchored to its corresponding bone section 67, 68 using any one ofthe three anchoring assemblies shown in FIGS. 4, 5 and 5A. Each of theseanchoring assemblies may also be used to anchor a bone plate component82 of any device to bone not just that of a distraction osteogenesisdevice.

Referring to FIG. 5, another anchoring assembly 83 may be used to attachbone plate 80 or any other bone plate component 82 to bone. Anchoringassembly 83 is a modified version of anchoring assembly 70 in which aheaded coupling pin 84 is used with a threaded shank 85 and a boneanchor 86 with a threaded bore 87. The threaded portion 85 of pin 84 ismade of a shape memory material having a TTR. The threaded portion 85has been pre-programmed to have an outer dimension or diameter smallerthan the inside dimension or diameter of threaded bore 87 when at atemperature below its TTR, thereby enabling the coupling pin 84 to beeasily inserted and extracted from bore 87 with substantially zeroinsertion and extraction forces and without having to screw or unscrewpin 84 from bone anchor 86. The threaded shank 85 is also trained tohave an outer diameter larger than the inner diameter of threaded bore87 when at a temperature above its TTR. The threads on shank 85 and inbore 87 are preferably designed to mate with one another when thethreaded shank 85 is in its enlarged condition (i.e., above its TTR). Inorder to facilitate interlocking between the plate 82 and coupling pin84, similar matching threads may be employed. In this way, the couplingpin 84 may be made more resistant to being pulled out of bone anchor 86.Optionally, the threaded shank 85 may be modified by being splitlongitudinally into two or more sections 88 in order to facilitateenlargement of the outer diameter of shank 85, similar to that shown inFIG. 2C. In addition, coupling pin 84 may include a keyed head 89 incase full disengagement between threaded shank 85 and threaded bore 87does not occur (i.e., the outside diameter of shank 85 remains justslightly larger than the inside diameter of bore 87) at a temperaturebelow the TTR so that the threaded coupling pin 84 may still be insertedand removed. Such a loose fit would still allow insertion and removalwith only minimal applied force.

FIG. 5A illustrates an anchoring assembly 92 similar to the anchoringassembly 20 of FIGS. 1 and 2. In addition to the bone anchor or screw 26and the coupling pin 34, anchoring assembly 92 includes a headed femalereceptacle or stud 93. As with the anchoring assembly embodiment 20, theportion of coupling pin 34 disposed inside of the female receptacle 93of assembly 92 may or may not be made of shape memory material.Alternatively, this portion of pin 34 may be secured to the headed stud93 by some means other than using the shape memory effect. For example,they may be threaded, welded, brazed, or otherwise connected together.

Referring to FIGS. 6 and 6A, a bone plate 94, such as that used for adental bridge, may be attached to two sections 95 and 96 of bone usingtwo of the present anchoring assembly embodiments 99 in combination witha female receptacle in the form of a lockable ball joint 100. Eachanchoring assembly 99, like those previously described, includes a boneanchor 102 and a coupling pin 103. Each ball joint 100 includes an outercasing 105, which could be formed by part of the plate 94, and a splitball 106 having at least one split 107 and a cavity or bore 108 formedtherethrough. Bore 108 is adapted to receive a portion 110 of thecoupling pin 103. With pin 103 trained in the same manner as pin 34 (seeFIGS. 1, 2 and 2A), the thickening of pin 103 at temperatures above itsTTR causes the split ball 106 to expand and lock inside of casing 105.In this way, the relative orientation of plate 94 and bone sections 95and 96 may be stabilized. Split 107 facilitates the expansion of ball106 in reaction to the thickening of portion 110 of coupling pin 103. Itmay be desirable to use a split ball 106 with multiple splits 107 tofacilitate the transfer of forces from pin 103 even more.

Referring to FIGS. 7 and 8, the present invention is applicable to ananchoring assembly 114 for attaching a body part, such as soft tissue,(not shown) to bone 115 using a suture 118 (shown in phantom). Theassembly 114 includes a bone anchor or screw 119 having an opening 120leading to a stepped bore 123 formed therethrough. The bore 123 has aninitial section 124 with a hexagonal shape adapted for receiving anallen wrench 125 with a mating hexagonal cross-section. Bore 123 alsoincludes a deeper section 126 which is cylindrically shaped. A couplingpin 130 is adapted for being inserted into the bore 123 of screw 119.Pin 130 includes a cylindrical shank 131, and a hexagonally shaped head132 having a cross-section matching that of the allen wrench 125. Thecoupling pin 130 also includes an eyelet 133 formed above the hexagonalhead 132 for threadably receiving the suture 118. At least the shankportion 131 of pin 130 is made of a shape memory material having a TTR.The shape memory shank 131 and hexagonal head 132 may be fixed togetherin a number of ways. For example, shank 131 and head 132 may bethreadably engaged, welded or otherwise bonded together, or they may beof one piece construction from the same shape memory material. They mayalso be fixed together using the shape memory effect and structurepreviously described for the pin 34 and stump 35 (see FIGS. 1 and 2) andthe pin 34 and headed stud 94 (see FIG. 5A). The superelastic propertiesof the shape memory material may also be used to attach shank 131 andhead 132 together using well known snap fit or other conventionalsystems.

Thus, the anchoring assembly 114 may be installed by first drilling ablind bore 137 into the bone 115 and embedding (i.e., screwing) the bonescrew 119 into the bore 137 to an appropriate depth using tool 125.Preferably, the bone screw 119 is then left unloaded within bore 137 inorder to promote growth and bonding of the bone to the outer surface ofscrew 119. After a sufficient amount of time has passed, pin 130 issecured to screw 119. With shank 131 having been trained in the samemanner as the previously described pin 34, at least the shape memoryportion 131 of pin 130 is cooled to a temperature below its TTR suchthat the outer diameter of shank 131 is smaller than the inside diameterof the bore section 126. Pin 130 is then inserted into stepped bore 123with shank 131 loosely fitting within bore 126 and head 132 seated insection 124. The loose fit enables the pin 130 to be inserted into bore123 with substantially zero force. Shank 131 is then allowed to warm upto a temperature above its TTR, thereby causing it to change shape andlock inside of bore section 126 in the same manner as has been describedherein for previous embodiments. Optionally, shank 131 may be configuredand trained to respond in the same manner as that described for andillustrated in FIGS. 2B-D. With the coupling pin 130 now locked in placewithin the anchoring screw 119 (see FIG. 7), the body part such as aligament (not shown) may be anchored to the bone 115 using the suture118. The mating hexagonal shapes of head 132 and bore section 124 helpto prevent the coupling pin 130 from rotating and twisting out of bore123.

Referring to FIGS. 9A and 9B, instead of the coupling pin 130 witheyelet 133, an alternative coupling pin 140 may be used with bone anchor119. Pin 140 is basically a headless version of pin 130 with twolongitudinal slots or bores 141 and 142 formed along its length. Eachslot 141 and 142 is dimensioned to allow the passage of the suture 118therethrough. Pin 140 is made of a shape memory material having a TTRand has been similarly dimensioned and trained as the shank 131 of theprevious coupling pin 130. Therefore, pin 140 may be locked within screw119 and a body part (not shown) anchored in place with the suture 118using the same cooling and warming technique previously described.

Referring to FIGS. 10A-C, a number of other coupling pin designs may besuitable for use with a bone anchor, like screw 119. One such couplingpin 144 has a tubular section 145 with a longitudinal split 146 alongits length formed through to its central bore 147. The shape memory pin144 is trained so that its split 146 is closed enough at temperaturesbelow its TTR to permit the section 145 to be inserted through boresection 123. Pin 144 is further programmed such that its split 146opens, as shown in FIG. 10B, when it is allowed to warm unconstrained toa temperature above its TTR outside bore section 123. Central bore 147is dimensioned to permit the suture 118 to be threaded therethrough. Aspreviously described for other embodiments, the outer diameter of thetubular section 145 of pin 144 is smaller than the inside diameter ofbore section 126 at temperatures below the TTR and larger than theinside diameter of bore section 126 at temperatures above the TTR. Pin144 may then be locked within screw 119 by cooling and then warming pin144 in the same manner as described for the previous coupling pinembodiments 130 and 140. With having been previously threaded throughcentral bore 147, suture 118 may be secured to pin 144 by tying a knot148 at its end before pin 144 is inserted into the screw 119. In orderto increase the versatility of the suture anchoring assembly 114, pin144 may include a head 149 with the split 146 and bore 147 also beingformed therethrough. With the head 149, the suture anchoring assembly114 could serve another purpose, such as the same applications shown inFIGS. 4, 5 and 5A. Optionally, pin 144 may be designed without split 146(see FIG. 10C). In which case, tubular section 145 would simply becomethinner and thicker in the same manner as previously described for pin34 (see discussion for FIGS. 1, 2 and 2A-E). In addition, instead ofusing a tubular construction for the coupling member of suture anchoringassembly 114, other shapes may be desirable. For example, a shape memorysphere or ball may be used instead of pin 140 or 144. Such a ball mayhave one or more bores through which the suture 118 may be threaded inthe same manner as shown in FIGS. 9A and 9B and 10A-C. A shape memoryring may also be used instead of pin 140 or 144. Such a ring may have anoval or otherwise flattened shape at lower temperatures so that it maybe inserted into bore section 126 and a circular shape, if unconstrainedby bore section 126.

Referring to FIGS. 11 and 11A, the principles of the anchoring assembly20 (see FIGS. 1 and 2) may be combined with those of the lockable balljoint 100 (see FIGS. 6 and 6A) in order to produce another anchoringassembly embodiment 152 of the present invention. Assembly 152 includesa bone anchor or screw 154 and a shape memory coupling member or pin 155adapted to be received within bone screw 154 in the same manner as thatpreviously described for screw 26 and pin 34 of assembly 20. Assembly152 also includes a split ball 156 and a stump 158. Split ball 156 isstructurally similar to split ball 106. Split ball 156 is adapted toreceive pin 155 in the same manner that ball 106 receives pin 103 inanchoring assembly 99. Stump 158 is adapted for mounting a prostheticbody part such as a tooth 159. Stump 158 functions like outer casing 105in that stump 158 has a cavity 160 adapted to receive split ball 156.Thus, stump 158 is slidable around ball 156 and may be tilted relativeto pin 55 and screw 54 at least at temperatures below the TTR of theshape memory pin 155. The position of stump 158 relative to pin 155 inscrew 154 is fixed at least at temperatures above the TTR of shapememory pin 155. Thus, prosthetic body part 159 may be tilted at anydesirable angle relative to coupling member 155 and bone anchor 154 andfixed at that angle in the same manner as previously described for plate94 and anchoring assembly 99. Alternatively, instead of using stump 158,a prosthetic body part 163 could be provided with a cavity 164 adaptedto receive split ball 156 in the same manner as cavity 160 (see FIG.11A). In either case, the position of the body part 159 and 163 relativeto their respective bone anchors 154 may be adjusted or fixed byrespectively cooling or warming the corresponding coupling member 155according to the principles of the present invention, as previouslytaught herein.

From the above disclosures of the general principles and detaileddescription of exemplary embodiments incorporating principles of thepresent invention, those skilled in the art will readily appreciate thevarious changes and modifications to which the present invention issusceptible. Therefore, the scope of the present invention should belimited only by the following claims and equivalents thereof.

What is claimed is:
 1. An anchoring assembly for attaching a body partto bone comprising:a bone anchor having a first opening leading to afirst cavity formed therein and being implantable in the bone; acoupling member having one portion made of a first shape memory materialwith a first transformation temperature range and being movable throughthe first opening and in and out of the first cavity of said bone anchorwhen at a temperature below the first transformation temperature range,and while in the first cavity, the one portion of said coupling memberbeing locked to said bone anchor when at a temperature above the firsttransformation temperature range, said coupling member being attachableto the body part and said coupling member being insertable into andextractable from the first cavity of said bone anchor with substantiallyzero insertion and extraction forces, respectively, when the one portionof said coupling member is at a temperature below the firsttransformation temperature range, the one portion of said couplingmember having an aperture formed therethrough, the aperture beingsubstantially narrow when the one portion of said coupling member is ata temperature below the first transformation temperature range and,while outside of the first cavity, being substantially wide when at atemperature above the first transformation temperature range.
 2. Ananchoring assembly for attaching a body part to bone comprising;a boneanchor having a first opening leading to a first cavity formed thereinand being implantable in the bone: a coupling member having one portionmade of a first shape memory material with a first transformationtemperature range and being movable through the first opening and in andout of the first cavity of said bone anchor when at a temperature belowthe first transformation temperature range, and while in the firstcavity, the one portion of said coupling member being locked to saidbone anchor when at a temperature above the first transformationtemperature range, said coupling member being attachable to the bodypart and said coupling member being insertable into and extractable fromthe first cavity of said bone anchor with substantially zero insertionand extraction forces, respectively, when the one portion of saidcoupling member is at a temperature below the first transformationtemperature range, the one portion of said coupling member being forkedwith at least two prongs, the prongs being substantially closed when theone portion of said coupling member is at a temperature below the firsttransformation temperature range and while outside of the first cavity,being substantially open when at a temperature above the firsttransformation temperature range.
 3. An anchoring assembly for attachinga body part to bone comprising:a bone anchor having a first openingleading to a first cavity formed therein and being implantable in thebone; a coupling member having one portion made of a first shape memorymaterial with a first transformation temperature range and being movablethrough the first opening and in and out of the first cavity of saidbone anchor when at a temperature below the first transformationtemperature range, and while in the first cavity, the one portion ofsaid coupling member being locked to said bone anchor when at atemperature above the first transformation temperature range, saidcoupling member being attachable to the body part and said couplingmember being insertable into and extractable from the first cavity ofsaid bone anchor with substantially zero insertion and extractionforces, respectively, when the one portion of said coupling member is ata temperature below the first transformation temperature range; saidconnector including a female receptacle having a second opening leadingto a second cavity formed therein and another portion of said couplingmember being securable inside the second cavity of said femalereceptacle; the other portion of said coupling member securable to saidfemale receptacle being made of a second shape memory material having asecond transformation temperature range and being movable through thesecond opening and in and out of the second cavity of said femalereceptacle when at a temperature below the second transformationtemperature range, and while in the second cavity, the other portion ofsaid pin being locked to said female receptacle when at a temperatureabove the second transformation temperature range.
 4. The anchoringassembly of claim 3, said female receptacle being a ball joint having anouter casing and a split ball mounted for sliding in said casing, saidsplit ball forming the second opening and the second cavity of saidfemale receptacle, wherein with the other portion of said couplingmember being in said second cavity, said split ball being slidablerelative to said outer casing when the other portion of said couplingmember is at a temperature below the transformation temperature rangeand said split ball being fixed in place relative to said outer casingwhen the other portion of said coupling member is at a temperature abovethe second transformation temperature range.
 5. An anchoring assemblyfor attaching a body part to bone comprising:a bone anchor having afirst opening leading to a first cavity formed therein and beingimplantable in the bone; a coupling member having one portion made of afirst shape memory material with a first transformation temperaturerange and being movable through the first opening and in and out of thefirst cavity of said bone anchor when at a temperature below the firsttransformation temperature range, and while in the first cavity, the oneportion of said coupling member being locked to said bone anchor when ata temperature above the first transformation temperature range, saidcoupling member being attachable to the body part and said couplingmember being insertable into and extractable from the first cavity ofsaid bone anchor with substantially zero insertion and extractionforces, respectively, when the one portion of said coupling member is ata temperature below the first transformation temperature range: thefirst cavity of said bone anchor having an inside diameter and beingthreaded, the one portion of said coupling member being a threaded shankhaving an outer diameter, the outer diameter of said shank being smallerthan the inner diameter of the first cavity when said shank is at atemperature below the first transformation temperature range and theouter diameter of said shank being larger than the inner diameter of thefirst cavity when said shank is at a temperature above the firsttransformation temperature range, said shank being split longitudinally.6. An anchoring assembly for attaching a body part to bone comprising:abone anchor having a first opening leading to a first cavity formedtherein and being implantable in the bone; a coupling member having oneportion made of a first shape memory material with a firsttransformation temperature range and being movable through the firstopening and in and out of the first cavity of said bone anchor when at atemperature below the first transformation temperature range, and whilein the first cavity, the one portion of said coupling member beinglocked to said bone anchor when at a temperature above the firsttransformation temperature range, said coupling member being attachableto the body part and said coupling member being insertable into andextractable from the first cavity of said bone anchor with substantiallyzero insertion and extraction forces, respectively, when the one portionof said coupling member is at a temperature below the firsttransformation temperature range; said connector including a femalereceptacle having a second opening leading to a second cavity formedtherein and another portion of said coupling member being securableinside the second cavity of said female receptacle; said femalereceptacle being a stud having an eyelet formed therethrough forreceiving a suture thread.
 7. An anchoring assembly for attaching a bodypart to bone comprising:a bone anchor having a first opening leading toa first cavity formed therein and being implantable in the bone; acoupling member having one portion made of a first shape memory materialwith a first transformation temperature range and being movable throughthe first opening and in and out of the first cavity of said bone anchorwhen at a temperature below the first transformation temperature range,and while in the first cavity, the one portion of said coupling memberbeing locked to said bone anchor when at a temperature above the firsttransformation temperature range, said coupling member being attachableto the body part and said coupling member being insertable into andextractable from the first cavity of said bone anchor with substantiallyzero insertion and extraction forces, respectively, when the one portionof said coupling member is at a temperature below the firsttransformation temperature range; said coupling member having an eyeletformed therethrough for receiving a suture thread.
 8. An anchoringassembly for attaching a body part to bone comprising:a bone anchorhaving a first opening leading to a first cavity formed therein andbeing implantable in the bone; a coupling member having one portion madeof a first shape memory material with a first transformation temperaturerange and being movable through the first opening and in and out of thefirst cavity of said bone anchor when at a temperature below the firsttransformation temperature range, and while in the first cavity, the oneportion of said coupling member being locked to said bone anchor when ata temperature above the first transformation temperature range, saidcoupling member being attachable to the body part and said couplingmember being insertable into and extractable from the first cavity ofsaid bone anchor with substantially zero insertion and extractionforces, respectively, when the one portion of said coupling member is ata temperature below the first transformation temperature range; and aconnector interconnecting said coupling member and the body part; saidconnector including a suture secured to said coupling member.
 9. Ananchoring assembly for attaching a body part to bone comprising:a boneanchor having a first opening leading to a first cavity formed thereinand being implantable in the bone; a coupling member having one portionmade of a first shape memory material with a first transformationtemperature range and being movable through the first opening and in andout of the first cavity of said bone anchor when at a temperature belowthe first transformation temperature range, and while in the firstcavity, the one portion of said coupling member being locked to saidbone anchor when at a temperature above the first transformationtemperature range, said coupling member being attachable to the bodypart and said coupling member being insertable into and extractable fromthe first cavity of said bone anchor with substantially zero insertionand extraction forces, respectively, when the one portion of saidcoupling member is at a temperature below the first transformationtemperature range; the one portion of said coupling member being a tube,said tube having a longitudinal split.
 10. A method of attaching a bodypart to bone, comprising the steps of:anchoring a bone anchor to thebone, the bone anchor having an opening leading to a cavity formedtherein; cooling one portion of a coupling member, made of a shapememory material with a transformation temperature range, to atemperature below the transformation temperature range; inserting thecooled one portion of the coupling member through the opening and intothe cavity of the bone anchor with substantially zero insertion force;locking the coupling member in the cavity of the bone anchor by bringingthe one portion of the coupling member to a temperature above thetransformation temperature range; and attaching the coupling member tothe body part by suturing.
 11. The method of claim 10, said methodincluding the step of removing the coupling member from the cavity bycooling the one portion.
 12. The method of claim 10 including the stepof removing the coupling member from the bone anchor with substantiallyzero extracting force.